1. Field of the Invention
This invention relates to an improved packing element designed for use in completion and production operations of oil and gas wells wherein the expandable element is coated against exposure to hydrocarbons at elevated temperatures which prevents the element from unduly distending or elongating prior to its being sealed within the well bore.
2. Description of the Prior Art
Typically, expandable packing elements such as plug assemblies, bridge plugs, drillable packers, inflatable packers, and rotational locking sealing packers are used in subterranean wells in combination with various types of packer assemblies which are selectively located within a well casing in order to isolate one or more of the production zones of the well. Such packing elements are mounted within a packer assembly at the well head and the entire unit is run down into the well casing and secured at a selected location along the casing, normally adjacent production formations. When it is desired to operate the packer assembly to release a sealing plug or distend a packing element, a tubing string having a suitable actuator attached to its lower end is run down into the well casing to contact the plug or packing assembly, normally by applying sufficient downward force to the plug or packer assembly, the plug becomes disengaged from the packer assembly and free falls to the bottom of the well, such as in the form of an expendable plug, or the packer assembly is operated to distend the packing element radially to seal a well annulus, for example.
During the movement of tubing into the lowermost extremities of tuhe well, the sealing elements of packer apparatus during completion and production operations are subjected to high temperature and high pressure in oil and gas wells which has caused preliminary damage or deterioration of the sealing systems which utilize elastomeric packing elements. Damage to such elements has become a greater problem during present day intensive searching for new oil and gas reserves wherein the drilling and subsequent completion is being effected in deeper wells involving greater exposure to extremely hostile, high temperature environments where the well production may contain not only the desired hydrocarbons but significant amounts of hydrogen sulfide, carbon dioxide and methane, all of which are detrimental to elastomeric materials at elevated temperatures.
To overcome these conditions and successfully complete such a well the packer apparatus, including its elastomeric expandable packing element, must be capable of continuous sealing integrity and must be protected from damage from the aforesaid adverse environment prior to its sealing disposition in the desired location. The packing element must be resistant to the well environment, i.e. temperature, pressure, well fluids, and the like, but also to physical stresses imposed on the packing assembly during or resulting from completion or workover procedures.
One type of prior art seal system is disclosed in U.S. Pat. No. 2,862,563 illustrating a well packer apparatus for packing the annular space between tubing in a well wherein resilient annular packing elements are spaced about a tubular mandrel. U.S. Pat. No. 3,083,785 discloses the use of a formation packer in which a plurality of resilient annular packing elements are spaced about a tubular mandrel and a plurality of folded metal plates are mounted on a double traveling mandrel. U.S. Pat. No. 3,531,236 discloses a tubular sealing assembly utilizing chevron-shaped sealing rings formed from a fluoroelastomer and asbestos with a fluorocarbon plastic ring adapters at each end of the seal stack. U.S. Pat. No. 2,467,822 discloses the use of a rubber or similar packing material which is prevented from flowing through the opening between the packer body and the packing retainer or abutment surrounding the body.
The prior art also discloses a number of generic sealing systems having utility in the sealing of a well conduit. U.S. Pat. No. 3,467,394 discloses a packing element of a V-ring type wherein the packing arrangement comprises a polytetrafluoroethylene commonly sold under the trademark "Teflon" with relatively rigid V-ring shaped spacer rings interposed between a plurality of elastomeric V-rings. Also U.S. Pat. No. 4,050,701 discloses ring seals obtained from a mixture of polyphenylene sulfide and polytetrafluoroethylene for use in the fluid sealing of rotary or reciprocating shafts. Additionally, U.S. Pat. No. 3,626,337 discloses a packing ring for use in high temperature and high pressure environments wherein the thermoplastic type composition, such as rubberized nylon, tetrafluoroethylene polyesters, acrylics and the like, are laminated to form a composite sealing material. U.S. Pat. No. 3,799,454 discloses a coating composition containing polytetrafluoroethylene and polyethylene sulfide for formation of a seal system.
In general, the sealing systems of the prior art have not been totally satisfactory for use in modern-day wells having high bottom hole temperatures and pressures as well as containing corrosive fluids. Various types of newly available elastomeric materials have been utilized in packer seal systems, such elastomeric materials such as polytetrafluoroethylene sold under the trademark "Teflon", a polymer of polyphenylene sulfide sold under the trademark "Ryton", and a perfluoroelastomer sold under the trademark "Kalrez".
Polytetrafluoroethylene is a flexible fluoropolymer having a high degree of permanent set and cold flow exhibiting high resistance to corrosive chemicals and high temperatures. It is frequently used in combination with suitable fillers to improve its properties, especially resistance to high temperatures. Polyphenylene sulfide is a thermoplastic resin which exhibits high thermal stability, excellent chemical resistance, and good affinity for retaining fillers. The perfluoroelastomer is another material characterized by high thermal stability and excellent chemical resistance. All of the aforesaid elastomeric materials have been employed in packer seal systems but not with complete success under all conditions in deeper wells. It has been found that sealing systems which incorporate such elastomeric materials have a definite tendency to adhere or stick to the well conduit when exposed to high temperatures when the sealing system must be retrieved from the well or be relocated to a different position.
Sticking of the seal system which can occur in multiple seal assemblies, for example, is not the only problem inherent with the aforesaid elastomers, but ease of fabrication and expense require that the elastomer material be one capable of resisting both steam and hydrocarbons when used in both geothermal and hydrocarbon wells without undue swelling or elongation of the material prior to its controlled expansion or distension into sealing relation. In some cases the running in of the packer assembly may require periods as long as 24 hours and the packing element may be subjected to temperatures ranging from 0.degree. to 600.degree. F. Longer time periods may occasionally be required where the packer assembly must be capable of withstanding several trips into the well prior to setting the packing element.